The present invention relates to a novel process for the separation of optically active D- or L-.alpha.-phenylglycine hydrochloride from a supersaturated solution of DL-.alpha.-phenylglycine hydrochloride by direct optical resolution.
In the optical resolution of DL-.alpha.-phenylglycine for many years it has been commercial practice to form diastereomers with d-camphor-sulfonic acid. Further, in fractional crystallization it is known to form ammonium salts (Japanese Patent Publication No. 45,383/74) or monoethylamine salts (Japanese Patent Laid-Open Application No. 29,715/73) of N-acetyl-.alpha.-phenylglycine. These prior art methods, however, have serious drawbacks in that the commercial method employs d-camphor which is an expensive natural substance, while the latter methods take a complicated route, that is, an .alpha.-phenylglycine racemate is converted into an acetyl derivative which is resolved into two isomers, each of which must, in turn, be subjected to hydrolysis of the acetyl groups.
In general, when an .alpha.-amino acid is to be optically resolved by fractional crystallization, it is, of course, advantageous to treat the amino acid as directly as possible. However, attempts to selectively crystallize isomers of an amino acid in the form of the free amino acid, simple salts of a mineral acid or metal salts have rarely met with success. Japanese Patent Laid-Open Application No. 14,645/70 discloses the purification of a salt of optically active .alpha.-phenylglycine with a mineral acid on the basis of the differing solubilities of salts of the mineral acid the optically active isomer and a DL-mixture (a racemate). Since this publication discloses that the resulting mineral acid salt of .alpha.-phenylglycine is still a racemic compound, this method does not lead to resolution by fractional crystallization, but to only purification.